1. Field of the Invention
This invention relates to testing of oil and gas wells, and more particularly, to methods and apparatus for obtaining a fluid sample after flowing fluid into a surge chamber to obtain good drawdown of pressure in the well.
2. Description of the Prior Art
During the testing and completion of oil and gas wells, it is of ten necessary to test or evaluate the production capabilities of the well. This is typically done by isolating a subsurface formation or a portion of a zone of interest which is to be tested and subsequently flowing a sample of well fluid either into a surge chamber or up through a tubing string to the surface. Various data, such as pressure and temperature of the producing well fluids, may be monitored downhole to evaluate the long-term production characteristics of the formation.
One very commonly used well testing procedure is to first cement a casing into the borehole and then to perform the testing adjacent zones of interest. Subsequently, the well is flow tested through perforations in the casing. Such flow tests are commonly performed with a drill stem test string which is a string of tubing located within the casing. The drill stem test string carries packers, tester valves, circulating valves and the like to control the flow of fluids through the drill stem test string.
Although drill stem testing of cased wells provides very good test data, it has the disadvantage that the well must first be cased before the test can be conducted. Also, better reservoir data can be obtained immediately after the well is drilled prior to casing the well and before the formation has been severely damaged by drilling fluids and the like.
For these reasons alone, it is often desirable to evaluate the potential production capability of a well without incurring the cost and delay of casing the well. This has led to a number of attempts at developing a successful open-hole test which can be conducted in an uncased borehole.
One approach which has been used for open-hole testing is the use of a weight-set, open-hole compression packer on a drill stem test string. To operate a weight-set, open-hole compression packer, a solid surface must be provided against which the weight can be set. Historically, this is accomplished with a perforated anchor which sets down on the bottom. Another prior art procedure for open-hole testing is shown in U.S. Pat. No. 4,246,964 to Brandell, assigned to the assignee of the present invention. The Brandell patent is representative of the system marketed by the assignee of the present invention as the Halliburton Hydroflate System. The Hydroflate System utilizes a pair of spaced inflatable packers which are inflated by a downhole pump. With either of these devices, both of which have advantages and disadvantages, well fluids can then flow up the pipe string which supports the packers in the well.
Another approach to open-hole testing is through the use of pad-type wireline testers which simply press a small resilient pad against the side wall of the borehole and pick up samples through an orifice in the pad. An example of such a pad-type tester is shown in U.S. Pat. No. 3,577,781 to LeBourg. The primary disadvantage of pad-type testers is that they often take a very small unidirectional sample which is often not truly representative of the formation because it is "dirty" fluid which provides very little data on the production characteristics of the formation. It is also sometimes difficult to seal the pad. When the pad does seal, it is subject to differential sticking and sometimes the tool may be damaged when it is removed.
Another shortcoming of wireline formation testers which use a pad is that the pad is relatively small. If the permeability of the formation is high, hydrostatic pressure can be transmitted through the formation between the outside of the pad and the center of the pad where the pressure measurement is being made, in a very short period of time. This will result in major hydrostatic pressures soon after attempting to measure formation pressure. This may limit the effectiveness of wireline formation testers in some conditions.
The methods and apparatus of the present invention solve these problems by providing for flowing formation fluid into a surge chamber which is placed in communication with the formation or zone of interest by a pressure-actuated valve. This prevents the capturing of "dirty" fluid which initially comes out of the formation or zone of interest, while allowing capturing of a sample of the cleaner, more representative fluid flowing behind the dirty fluid.
Another approach which has been proposed in various forms is to provide an outer tubing string with a packer which can be set in a borehole, and in combination with a wireline-run surge chamber which is run into engagement with the outer string so as to take a sample from below the packer. One example of such a system is shown in U.S. Pat. No. 3,111,169 to Hyde, and assigned to the assignee of the present invention. Other examples of such devices are seen in U.S. Pat. No. 2,497,185 to Reistle, Jr.; U.S. Pat. No. 3,107,729 to Barry, et al.; U.S. Pat. No. 3,327,781 to Nutter; U.S. Pat. No. 3,850,240 to Conover; and U.S. Pat. No. 3,441,095 to Youmans. A disadvantage, obviously, is the extra time necessary to run-in and position the surge chamber.
A number of improvements in open-hole testing systems of the type generally proposed in U.S. Pat. No. 3,111,169 to Hyde are shown in U.S. Pat. No. 5,540,280 to Schultz et al., assigned to the assignee of the present invention. In a first aspect of the invention of U.S. Pat. No. 5,540,280, a system is provided including an outer tubing string having an inflatable packer, and a communication passage disposed through the tubing string below the packer, an inflation passage communicated with the inflatable element of the packer, and an inflation valve controlling flow of inflation fluid through the inflation passage. The inflation valve is constructed so that the opening and closing of the inflation valve is controlled by a surface manipulation of the outer tubing string. Thus, the inflatable packer can be set in the well simply by manipulation of the outer tubing string and applying fluid pressure to the tubing string without running an inner well tool into the tubing string. After the packer has been set, an inner well tool, such as a surge chamber, may be run into and engaged with the outer tubing string to place the inner well tool in communication with a subsurface formation through the communication passage. There is also an embodiment with a straddle packer having upper and lower packer elements which are engaged on opposite sides of the formation.
In another aspect of this prior invention, the well fluid samples are collected by running an inner tubing string, preferably an inner coiled tubing string, into the previously described outer tubing string. The coiled tubing string is engaged with the outer tubing string, and the bore of the coiled tubing string is communicated with a subsurface formation through the circulation passage defined in the outer tubing string. Then well fluid from the subsurface is flowed through the communication passage and up the coiled tubing string. Such a coiled tubing string may include various valves for control of fluid flow therethrough. This prior invention does not include the use of a surge chamber or sampler downhole to obtain the fluid sample.
In the present invention, a closure valve is utilized in the apparatus to open the surge chamber. The valve is actuated by pressure. A sampler in communication with the surge chamber is used to obtain a sample, and electronic pressure and/or temperature recording instruments may also be used to record fluid characteristics.